In telephone switching systems, a line circuit commonly serves as an interface between the telephone switching network and a communication line. A subscriber station set or a private branch exchange (PBX) commonly terminates the other end of the line. Traditionally, line circuits include a battery feed circuit that supplies current to the line. Line circuits also detect the on-hook and off-hook conditions of the line. Various sequences of these two line conditions are used for control signaling to request service and to provide well-known address and supervisory information. There are many different types of control signaling schemes that may be used depending on the type of line and terminal equipment.
To reduce an annoying 60-hertz signal commonly induced on the line, the impedance of the communication line conductors may be electrically balanced with respect to ground. This may be accomplished by electrically isolating the line from ground through the use of a floating or transformer-coupled battery feed circuit. The 60-hertz signal is the voltage between the conductors of the line having unequal impedances with respect to ground. This voltage, also known as metallic voltage, is the result of unequal longitudinal currents in the line conductors that are induced by electrical power lines in the vicinity of the communication line. With a balanced, electrically isolated line having a high AC impedance, the floating battery feed circuit does not unbalance the impedance of the line conductors with respect to ground.
The problem with electrically isolated lines is that they are incompatible with terminal equipment having supervisory signaling that requires a DC ground reference. Such is the case with a PBX having ground start supervisory signaling. Simply grounding one conductor of an electrically isolated line unbalances the impedance of the two line conductors. As a result, a 60-hertz signal is introduced on the line. Connecting a large inductor between one conductor of the line and ground provides a DC ground reference while keeping the line longitudinally balanced, but the large size of the required inductor is prohibitive. A drawback is that the large inductor can generate hazardous voltages on the line when the line current is interrupted.
In one prior art solution to another problem associated with electrically isolated lines, an anticorrosion biasing circuit keeps the polarity of the DC voltage on an electrically isolated line negative. This circuit is presently used with the electrically isolated lines of the 5ESS.TM. switch that is manufactured by AT&T Technologies, Inc. Positive polarity voltages cause the corrosion of telephone lines. Since the biasing circuit senses only positive polarity voltages from the battery feed circuit, negative polarity voltage fluctuations may cause false ground start supervisory signaling indications. Thus, this anticorrosion biasing circuit is not suitable for ground start supervisory signaling applications.